1. Field of the Invention
The present invention relates to a method for mounting a wire board on a liquid crystal display substrate so as to connect the display substrate to a semiconductor chip, a flexible wire board, and other surfaces. Hereinafter, the liquid crystal display will be referred to as "LCD."
2. Description of the Related Art
Under common practice a semiconductor chip for driving the liquid crystals in a display substrate is bonded face-down to the display substrate with an electrically-conductive adhesive or a solder.
In general, as shown in FIG. 6 an LCD apparatus is provided with an LCD substrate 21 and a counter glass substrate 31 bonded to each other with a liquid crystal layer (not shown) sandwiched therebetween. The LCD substrate 21 is provided with a wiring pad 22 and a connecting terminal 28 through which a semiconductor chip 24 and a flexible wire board 26 are connected to the LCD substrate 21.
A typical example of the known method of mounting the semiconductor chip 24 and the flexible wire board 26 on the LCD substrate 21 will be described by reference to FIG. 6:
The semiconductor chip 24 is provided with bump electrodes 23 on the undersurface thereof. The bump electrodes 23 are bonded to the wiring pads 22 and one of the connecting terminals 28 by use of an electrically-conductive adhesive 25. The reliability against water of the connecting points of the bump electrode 23 to the wiring pad 22 and the connecting terminal 28, respectively is low. Therefore, a thermosetting epoxy resin 30 is then injected through a gap between the counter glass substrate 31 and the semiconductor chip 24 until the epoxy resin 30 covers from a lower part of the semiconductor chip 24 up to the sides thereof due to capillarity. This method has advantages, in that the semiconductor chip 24 is embedded in the resin 30 and sealed to the substrate 21, and that on the side of the semiconductor chip 24 near the connecting terminal 28 a fillet 32 is formed. The fillet 32 prevents water from entering the connecting points through the interface between the epoxy resin 30 and the LCD substrate 21.
The thermo-setting epoxy resin 30 is preferably a type which exhibits good wettability with the LCD substrate 21. The length of the fillet 32 depends upon the amount of supply of the resin 30.
Subsequently, the flexible wire board 26 is bonded to that side of the connecting terminal 28 which is opposite to the semiconductor chip 24 through a thermoplastic layer 27 of anisotropic conductivity which permits reworking. Then, the connecting part 34 of the flexible wire board 26 is covered with a silicon resin coat 33 so as to protect it against water. In this way the mounting method is finished.
However, the above-mentioned known mounting method has disadvantages; one is that because of the use of a thermoplastic layer 27 of anisotropic conductivity, no heat can be applied after the flexible wire board 26 is bonded to the LCD substrate 21. Therefore, before the flexible wire board 26 is bonded to the substrate, the semiconductor chip 24 is sealed in epoxy resin 30. A fillet 32 is thus formed in which it is difficult to control the length. As a result, the connecting part 34 of the LCD substrate 21 and the flexible wire board 26 are detrimentally sealed in a flooded epoxy resin 30.
In order to avoid such problems, it is required to keep the pads 22 and the connecting part 34 of the flexible wire board 26 at a relatively large distance. This results in increasing the size of the LCD substrate 21.
A proposed solution to the flooding problem of epoxy resin 30 is a dam by means of a heat-proof tape in the connecting terminal. However, this method presents at least two problems; one is that the number of production steps is increased, and the other is that the remainder of the dam is likely to spoil the adherence of the silicon resin coat 33 with the connecting terminal 28. This leads to a poor electrical connection at the connecting part 34 of the flexible wire board 26.
In general, the silicon resin 33 is of two types; one is a condensation type resin (merely condensation resin) and the other is an addition type resin (merely addition resin). Addition silicon resins are superior to condensation silicon resins in antihumidity, but a problem is that the addition resin is difficult to harden sufficiently when it comes into contact with any resin containing an amine-type hardener.
In practise, since the thermoplastic layer 27 of anisotropic conductivity and the thermo-setting epoxy resin 30 both contain an amine-type hardener, an addition resin cannot be used, and as a substitute, the condensation type must be used. As a result, the favorable anti-humidity property of the addition resin cannot be utilized.